The isocyanate products have a value of widespread industrial application and are mainly classified into aliphatic isocyanates and aromatic isocyanates which can produce various polyurethane products by polymerization. In these products, the aromatic isocyanates have the greatest market demand. The aromatic polyisocyanates are very important raw materials and have been widely applied to the fields of synthetic leathers, fibers, coatings, plastics and so on. Recently, with the increased market demands of the domestic automobiles, leather production and construction industry, the demand of the aromatic polyisocyanates has been also rapidly expanded.
The approach of synthesizing the aromatic isocyanates can be achieved by various technical routes. However, the method currently used for the industrial production is mainly a phosgene process due to the economy thereof. Since the phosgene process has high toxicity and severe pollution due to use of hypertoxic phosgene raw material and is thus hazardous to the ambient ecology and environment, it has an extremely high requirement for the industrial mass production, which severely limits the enlargement of the production of the aromatic polyisocyanates and the widespread application of the downstream products. Therefore, there is an urgent demand for green processes of producing the aromatic isocyanate products. Moreover, the process of producing the aromatic polyisocyanates through the pyrogenation of polymethylene polyphenyl aromatic carbamate is most likely to be industrialized and is desirable to produce the aromatic polyisocyanate in place of the phosgene process.
In the green processes of synthesizing the aromatic polyisocyanates, the efficient synthesis of polymethylene polyphenyl aromatic carbamate is one of the critical determining factors for achieving the industrialization of the process. Polymethylene polyphenyl aromatic carbamate can be synthesized through various technical routes. In these technical routes, the production of polymethylene polyphenyl polycarbamate through the condensation of monophenyl carbamate and methylating reagent has been mostly investigated. It is generally a relatively complicated process to produce polymethylene polyphenyl polycarbamate through the condensation of monophenyl carbamate and methylating reagent. The resulting polymethylene polyphenyl polycarbamate is present in the form of various isomers and the product is a mixture of carbamates having various benzene ring numbers. In conventional condensation reaction processes, it is often necessary to use different solvents as the reaction medium according to the requirement of the reaction catalyst. The used catalyst is mainly classified into a liquid protonic acid and the liquid protonic acid catalyst mainly comprises a strong acid such as an inorganic acid (sulfuric acid, hydrochloric acid, hydrofluoric acid etc.) and an organic acid (formic acid, acetic acid, butyric acid etc.).
In the U.S. Pat. No. 4,307,029 (1981), Takeuchi et al. have reported a technical route for synthesizing polymethylene polycarbamate through the condensation of phenylcarbamate with formaldehyde or derivative thereof in an organic solvent under the catalyzation of a Lewis acid and a protonic acid, followed by the formation of the corresponding polyisocyanate mixture through pyrogenation. However, because the composition of the condensed product is complicated and the pyrogenating temperature is higher, the composition of the final product is more complicated, and the subsequent separation in the scale-up process is extremely complicated, bringing about the difficulty of industrialization thereof.
In the patent JP 01135758, Takeshita et al. have reported that the technical route for synthesizing methylene diphenyl di(ethyl carbamate) through the condensation of ethyl phenylcarbamate with formaldehyde under the catalyzation of sulfuric acid can achieve a conversion of ethyl phenylcarbamate of 98.4% and a yield of the product of 73%.
In the patent DD206669-A, Jungnickel has used 20% of hydrochloric acid as the catalyst and solvent and investigated the reaction for producing methylene diphenyl di(methyl carbamate) through the condensation of methyl phenylcarbamate and formaldehyde, wherein the selectivity of 4,4′-methylene diphenyl di(methyl carbamate) is 72% and the selectivity of 2,4′-methylene diphenyl di(methyl carbamate) is 7%.
In the patent JP 04202172, Muzakami has proposed that strong acids such as HCO2H, H3PO4 and H2SO4 are formulated into an aqueous mixed acid solution as the catalyst and solvent for the condensation of carbamate and formaldehyde, wherein the conversion of carbamate is 92% and the selectivity of the product is 88%.
Prof. Gongying Wang in the Chengdu Institute of Organic Chemistry of the Chinese Academy of Science has successively reported (Chemical Engineering of Natural Gas, 2004, volume (vol.) 29(2), pages (pgs.) 33-36; and Industrial Catalysis, 2006, vol. 14(8), pgs. 44-47) that methylene diphenyl di(methyl carbamate) is synthesized through the condensation in a system of a mixed acid and of sulfuric acid, wherein the reaction of producing methylene diphenyl di(methyl carbamate) through the condensation of methyl phenylcarbamate and formaldehyde solution (or trioxymethylene) in the presence of 30% of the mixed acid catalyst produces the following results: when formaldehyde is used as the methylating reagent, the yield of the product is 89.47%, and when trioxymethylene is used as the methylating reagent, the yield of the product is 75.81%. The reaction of producing methylene diphenyl di(methyl carbamate) through the condensation of methyl phenylcarbamate and formaldehyde by adding hydrochloride salt promoter to the system on the basis of the sulfuric acid catalyst has been further investigated and it has been found that the conversion of methyl phenylcarbamate is 96% and the yield of 4,4′-methylene diphenyl di(methyl carbamate) can be 76.7%, but the introduction of chloride ions to the reaction system will affect the usability of the product in the subsequent application.
Prof. Yanji Wang et al. have reported the reaction of catalytically synthesizing methylene diphenyl di(methyl carbamate) from methyl phenylcarbamate and formaldehyde in a system of a protonic acid-acidified 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim]BF4) ionic liquid (Journal of Chemical Engineering of Chinese Universities, 2007, vol. 21(3), pgs. 467-470) and of a sulfonic acid-functionalized ionic liquid (Chinese Journal of Chemical Engineering, 2009, vol. 17(05), pgs. 756-760). However, the yield of the product is less than 75%.
In the above patents and literatures, the condensation of phenylcarbamate has been investigated mainly by selecting and using the liquid and solid acid catalysts in a water-phase system, and the inexpensive and easily available formalin (aqueous formaldehyde solution) has been used as the methylating reagent during the production so that the production cost is low. However, there are the technical difficulties of corrosion of apparatus, the great amount of acid liquid used and the complicated post treatment processes such as the separation of the acid liquid from the solid polycyclic carbamate product in the production. Moreover, it is extremely difficult to wash off the acid liquid entrained by the solid product, which limits the industrial scale-up and application thereof.
Monica Distaso et al. (JP 58062151, JP 56167656 and US 1981-05-19) have used Sc(OTf)3 and La(OTf)3 (OTf=O3SCF3) as the catalyst and investigated the reaction where phenyl methyl carbonate is used as the carbonylating agent of the aromatic diamine. The methoxycarbonylating reaction of MDA and phenyl methyl carbonate in tetrahydrofuran organic solvent is markedly improved, wherein the overall yield of carbamate is approximately 80% and the selectivity is as high as 94%. However, the stabilization, separation, recycle and reuse of the above catalyst are technically problematic, which limits the further industrial application thereof. In the patent CN 101440 048, by introducing the mixed solvent system of acetic acid and water, the applicant has improved the yield of the product from the condensation of methyl phenylcarbamate and formaldehyde at a certain degree and relieved the problem of entraining liquid acid catalyst in the solid product. However, it has not disclosed that the presence of a proper amount of solvent allows the reaction to occur in the uniform liquid phase system and the crystallization of the solid product during the reaction to be completely avoided such that the technical difficulty of separating the product from the acid catalyst and recycle and reuse the product can be fundamentally solved.
In the above technical routes of synthesizing polymethylene polyphenyl polycarbamate, the reaction system has a complicated multiphasic phenomenon where the aqueous phase, oil phase and solid phase are co-existed and the solid phase product entrains the phenylcarbamate oil phase raw material and the catalyst, which results in the technical difficulty of complicated subsequent product separation and purification.